The already volatile special election campaign to replace Jesse Jackson Jr. in Congress saw its biggest upheaval Sunday when state Sen. Toi Hutchinson dropped out and endorsed former state Rep. Robin Kelly in a move to unite against former U.S. Rep. Debbie Halvorson.

Hutchinson, of Olympia Fields, had been considered a top candidate along with Kelly, Halvorson and Chicago Ald. Anthony Beale, 9th, among what had been 16 candidates in the special Feb. 26 Democratic primary that will all but determine the next congressman due to the heavily Democratic nature of the 2nd Congressional District.

Hutchinson's decision furthers the momentum of Kelly, a Matteson resident who left her job as a top aide to Cook County Board President Toni Preckwinkle to run for Congress. Preckwinkle had endorsed Hutchinson over Kelly, but Sunday signed on in support of her former employee and criticized Halvorson for opposing President Barack Obama on key initiatives.

Kelly said that while she and Hutchinson "haven't agreed on everything, we have a strong mutual respect, share a passion for public service and dedication to the people in Chicago" and the south suburbs. Kelly's campaign has been almost single-focused on gun control. Notably, Kelly did not criticize Halvorson in her statement, instead leaving that to others.

Sources said Hutchinson appeared to be having fundraising problems — despite reporting having nearly $200,000 in her campaign fund less than two weeks ago. Hutchinson became the latest target of attack TV ads from the wealthiest component of the race, New York Mayor Michael Bloomberg's anti-gun super political action committee. Hutchinson also was forced to defend what one news organization said were payments of $57,140 out of her state campaign fund to her mother for constituent services.

In announcing her withdrawal, Hutchinson said she was "simply unwilling to risk playing a role going forward that could result in dividing our community at a time when we need unity more than ever."

"In the wake of horrendous gun-related crimes all across our country, I agree with Robin that we need to stand together to fight gun violence, but Debbie Halvorson (has) been wrong headed in her refusal to moderate her views on banning dangerous assault weapons," Hutchinson said.

Gun violence has been a top theme of the contest. Halvorson, a former one-term congresswoman from Crete, has rejected calls for a ban on military-style assault weapons that Obama wants.

Hutchinson was Halvorson's chief of staff when Halvorson served in the Illinois Senate. When Halvorson won election to Congress in 2008, Hutchinson succeeded her in the legislature.

But Halvorson also is the leading white candidate in the Democratic field for a district that was drawn for minority representation. Halvorson previously has alleged that race has been a factor in the decision of some prominent Democrats to endorse her challengers.

Hutchinson's decision consolidates the field considerably behind Kelly, though Beale, an African-American and the only elected officeholder from Chicago in the contest, could be a factor.

In a statement, Halvorson contended a "behind the scenes" deal was cut between Kelly and Hutchinson that sought to deny voters of their "right to choose their representative."

"The voters of the 2nd Congressional District deserve better," Halvorson said. "These old political games are what got us in this mess in the first place."

Halvorson had been viewed as a leading contender, in part to the name recognition she acquired last year in challenging Jackson for the Democratic nomination, though she lost by a wide margin. She had encouraged a large field to compete on the Democratic ballot.

Shortly after Jackson stepped down in November amid a federal ethics investigation and diagnosis of bipolar depression, some leading black politicians called for unity around one African-American candidate to avoid splitting the vote and giving the seat to Halvorson.

On Friday, Jackson was charged by federal prosecutors with using campaign cash to enrich himself and his wife, former 7th Ward Ald. Sandi Jackson. Sandi Jackson was charged separately with understating the couple's income.

Hutchinson dropped out as Bloomberg's super PAC had begun a new round of TV ads that included criticism of her and Halvorson for having received previous backing from the National Rifle Association. Previously, the super PAC had focused its attention on attacking Halvorson, primarily through $1.4 million in the only broadcast ads in the race airing on Chicago TV.

Last week, Independence USA reported it was spending $730,336 for Chicago television time to also promote its endorsement of Kelly. That brings the super PAC's spending to more than $2.1 million in the race — by far more than any of the individual candidates have raised or spent.

Hutchinson had sought to defend her previous NRA support as reflecting the diversity of a legislative district that is part suburban and part rural. She said she decided to endorse Obama's gun-control initiatives and sponsor state versions of them in the aftermath of the school shootings in Newtown, Conn., because the NRA promoted arming school officials around the country.

Reflecting how events can overtake candidates' ability to control an accelerated special election campaign, Hutchinson's camp had figured a strong pro-ethics stance would be a place to plant a flag in a district that has seen its last three congressman leave office in scandal and disgrace.

Through Feb. 6, Hutchinson had reported raising $281,106 for the contest, and her $199,901 in cash left at that point was more than double the $88,820 that Kelly had reported. But Hutchinson's campaign also had embarked on an expensive direct-mail campaign since then.

Preckwinkle urged supporters to donate to Hutchinson's congressional campaign, both in emails and in phone calls, but her work appeared to come up short. Overall, Kelly reported raising $303,725 through Feb. 6, and her campaign said that total had grown to nearly $418,000 as of the middle of last week.